Geology
The Loess ( /ˈloʊ.əs/, /ˈlʌs/, or /ˈlɛs/) Hills are generally located between 1 and 15 miles (24 km) east of the Missouri River channel. The Loess Hills rise 200 feet above the flat plains forming a narrow band running north-south 200 miles along the Missouri River. These hills are the first rise in land beyond the flood plain, forming something of a "front range" for Iowa, and parts of Missouri and Nebraska adjacent to the Missouri River.
During the last Ice Age, glaciers advanced into the middle of North America, grinding underlying rock into dust-like "glacial flour." As temperatures warmed, the glaciers retreated and vast amounts of meltwater and sediment flooded the Missouri River Valley. The sediment was deposited on the flood plain, creating huge mud flats. When meltwaters receded, these mud flats were exposed. As they dried, the fine-grained silt was picked up by strong prevailing westerly winds. Huge dust clouds were moved and redeposited over broad areas. The heavier, coarser silt was deposited close to the Missouri River flood plain, forming vast dune fields. The dune fields were eventually stabilized by grass. Due to the erosive nature of loess soil and its ability to stand in vertical columns when dry, the stabilized dunes were eroded into the corrugated, sharply dissected bluffs we see today.
The dominant features of this landscape are "peak and saddle" topography, "razor ridges" (narrow ridges, often less than ten feet wide, which fall off at near ninety-degree angles on either side for 60 feet or more), and "cat-step" terraces (caused by the constant slumping and vertical sheering of the loess soil). The soil has a characteristic yellow hue and is generally broken down into several units based on the period of deposition (Loveland, Pisgah, Peoria). Loess is known locally as "sugar clay" because it can be extremely hard when dry, but when wet, loses all cohesion. The Loess Hills of Iowa are remarkable for the depth of the drift layer, often more than ninety feet deep. The only comparable deposits of loess to such an extent are located in Shaanxi, China.
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